Research has indicated that the tight junctional seal surrounding each epithelial cell in an epithelial tissue is compromised in the process of tumor formation. This has been shown by: 1) the ability of the tumor promoter class of secondary carcinogens to engender tight junctional leakiness through activation of protein kinase C (Mullin and O'Brien, 1986; Mullin et al., 1997); 2) the existence of leaky tight junctions between epithelia of human gastrointestinal tumors (Peralta Soler et al., 1999); and 3) the induction of tight junction leakiness in precancerous rat colon being exposed to primary carcinogens (Peralta Soler et al., 1999). The physiological implication of this leakiness is that it will compromise the barrier function of the entire epithelial tissue. This has in turn very important medical implications for the generation and progression of inflammatory and cancerous states (Mullin, 1998), particularly since proteins such as insulin have been demonstrated to cross these junctions intact (Mullin et al., 1998). Thus, precancerous and cancerous epithelial tight junctions will allow for diffusion of proteins and sugars from the lumen of the upper GI tract (esophagus and stomach) into the bloodstream. In addition however, the intrinsic compartmental physiology of epithelial tissues allows one to take advantage of a naturally occurring diagnostic indicator of leakiness in these tissues, an indicator which can provide a non-invasive early warning to cancerous and precancerous inflammatory states in epithelial tissues.
All epithelial cells in the body are polar, that is they possess distinct top and bottom surfaces. This structural polarity allows them to perform their two most basic functions: they can reabsorb substances in one direction or secrete other substances in the opposite direction. Secretion of acid into the stomach lumen or reabsorption of sugar from urine are both due to this structural polarity. This vectorial property holds true not just for acids, salts and sugars, but for proteins as well. Gastric epithelia generally secrete digestive enzymes such as trypsin into the lumen of the GI tract, not the bloodstream, whereas the hormones gastrin and secretin, are released in the opposite direction to enter the bloodstream (Mountcastle, Medical Physiology, 1974). This directionality is achieved by the structural polarity of the epithelial cells, but it is maintained by the tight junctional seals preventing back diffusion of substances across the epithelial barrier. As tight junctions become leaky in the process of development of epithelial cancer, backleak of these signature proteins will occur, causing their levels in the opposite fluid compartment to rise.
The present invention, relates to the early diagnosis of cancer by detection of a backleak of signature proteins in the gastrointestinal tract, considered as a continuum from the mouth to the rectum. Along this “tube” are various proteins and sugars which are vectorially secreted into the GI lumen and are specific not only to the GI tract but to specific sites in the GI tract. Salivary amylase (ptyalin) is a 55,000 molecular weight protein released by cells of the parotid gland into saliva, the first lumen of the GI tract (Dimagno, in Gastroenterology, 1980). It moves down the esophagus into the stomach simply with swallowing. Pepsinogens I and II (40,000 mw) are released into the lumen of the stomach from oxyntic glands. Once these proteins are exposed to the acidity of the lumen of the GI tract, they spontaneously form the smaller and catalytically active protein, pepsin (33,000 mw) (Mountcastle, Medical Physiology, 1974). This enzyme is functional in the stomach and upper intestine. In the lower intestine and colon, the trefoil factor, TFF3 or ITF, is secreted into the lumen. It is a 39 amino acid residue polypeptide which seems to be active in mucosal repair processes (Thim, 1997).
Tight junctional leakiness between gastrointestinal epithelia in the vicinity of the secretion of these proteins, or downstream of their secretion, will allow for their chronic leak into the bloodstream, raising their level in serum. Therefore, salivary amylase levels in serum have important diagnostic predictive value for esophageal and gastric precancerous conditions, specifically Barrett's Esophagus, atrophic gastritis and H. pylorii infection. Serum pepsin levels likewise have diagnostic value in precancerous gastric conditions, such as atrophic gastritis and H. pylorii infection. The secretion of TFF3 (ITF) in the lower intestine and colon makes its serum level predictive of precancerous leaks in the ileum and colon. For all three markers, elevated serum levels of these proteins can serve as low cost, noninvasive indicators whose presence can alert the physician to the need for the more expensive and involved endoscopic or colonoscopic follow-up procedures.
In addition to detecting leakage of signature proteins into the bloodstream, the present invention relates to detecting cancerous or precancerous conditions by leakage of signature carbohydrates from the epithelium into the bloodstream. Both leakage of signature proteins and leakage of signature carbohydrates serves as the basis for a noninvasive and relatively inexpensive screen for upper GI cancers and for precancerous and cancerous conditions throughout the GI tract. It would also serve to A similar approach could be utilized in a range of other epithelial tissues and cancers.